1. Field of the Invention
The present invention relates to a separator for a fuel cell and a method for anti-corrosion treatment of the same.
2. Description of the Related Art
The operation mechanism of a fuel cell begins by oxidizing a fuel, such as hydrogen, natural gas, and methanol, etc. to produce an electron and a hydrogen ion at the anode of the fuel cell. The hydrogen ion passes through an electrolyte membrane to the cathode and the electron is supplied to an outer circuit through a wire. The hydrogen ion which reaches the cathode is combined with the electron that reaches the cathode through the outer circuit and oxygen gas or oxygen gas in the air to form water.
Fuel cells are regarded as the next-generation energy conversion units since they have a high electricity generation efficiency and are environmentally friendly. Fuel cells are classified into categories including polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) depending upon the type of electrolyte used. Operation temperature, materials of the constitutional elements etc. can also vary depending on the type of fuel cell.
PEMFCs can be operated at relatively low operation temperatures, i.e., about 80 to 120° C. and have a very high current density, which makes them suitable as a power supply for vehicles and homes etc. The PEMFCs contain a bipolar plate, which is one of the main elements that require improvements to make the PEMFCs compact, light and economical.
The PEMFC comprises a bipolar plate and a membrane electrode assembly (MEA) as its main components. The MEA comprises an anode in which the fuel is oxidized, a cathode in which an oxidizing agent is reduced, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane has an ion conductivity sufficient to deliver a hydrogen ion generated in the anode to the cathode and an electronic insulation sufficient to electronically insulate the anode and the cathode.
It is well known in the art that the bipolar plate has channels for flowing fuels and air and that it also functions as an electron conductor for transporting electrons between MEAs. The bipolar plate should be non-porous such that the fuel and the air can be separated. In addition, it should have excellent electrical conductivity and have sufficient thermal conductivity to control the temperature of the fuel cell. Further, the bipolar plate should have a mechanical strength sufficient to bear a force applied at the time of clamping the fuel cell as well as corrosion-resistance against hydrogen ions.
In the past, graphite was used to make a bipolar plate in the PEMFC, and a channel for a fuel and air was mainly formed by milling. A graphite plate has a sufficient electrical conductivity and resistance to corrosion. However, the graphite plate and its milling process are very expensive. Further, the graphite plate is brittle and it is difficult to process the bipolar plate to a thickness of less than 2-3 mm. Due to a difficulty in decreasing the thickness of the bipolar plate, it is also difficult to decrease the size of a fuel cell stack consisting of several tens to several hundreds of unit cells.
In order to reduce the production costs and the thickness of the bipolar plate, an attempt was made to use a metal to form the bipolar plate. Metals have most of the physical properties necessary for the bipolar plate, and the material and production costs are both very economical. When compared to graphite plates, the costs of the bipolar plate is 1/100 or less of the price of the graphite plate.
However, the metallic bipolar plate may be eroded under the acidic conditions of a fuel cell. This could result in serious problems such as the membrane being poisoned and increased contact resistance. Corrosion of the metallic bipolar plate causes not only defects of the bipolar plate itself, but also electrolyte poisoning due to diffusion of metal ions into the electrolyte membrane. When the electrolyte is poisoned, the conductivity of a hydrogen ion of the electrolyte becomes decreased, thus resulting in deterioration of the performance of a fuel cell. Thus, the use of a metallic bipolar plate is inhibited by such corrosion of the metal.
In a 1000-hour performance test, a PEMFC using a bipolar plate made of, for example, stainless steel, a Ti alloy, or an Ni alloy has a lower performance than PEMFC using a graphite bipolar plate.
Research has been conducted aiming to improve the anti-corrosion of a metallic bipolar plate. Such efforts include a method of applying an anti-corrosive coating to the metallic bipolar plate. For example, a method of coating a material having excellent anti-corrosive effect and electrical conductivity, such as TiN, on a surface of a bipolar plate composed of Ti or stainless steel is disclosed in Korean Laid-Open Patent Publication No. 2003-0053406.
The above discussion relating to a bipolar plate can also be applied to an end plate, cooling plate, and a separator.
It is well known in the art that an end plate is an electronically conductive plate that has channels for a fuel or an oxidizing agent only on one side and is attached to MEAs disposed at both ends of a fuel cell stack, respectively.
It is well known in the art that a cooling plate is an electronically conductive plate that has channels for a fuel or an oxidizing agent on one side and channels of a cooling fluid on the other side.
It is well-known in the art that a separator is used when a flow field is formed in the diffusion layers of an anode and a cathode, and is generally understood as a bipolar plate without a flow field. Advantageously, the separator may have low gas permeability, excellent electrical conductivity, and excellent anti-corrosive effect.
The problems of the bipolar plate of PEMFC were described above, but such problems will also occur in MCFC, PAFC, DMFC, etc.